The present invention relates generally to cellular communication systems. More specifically, the present invention relates to adapting a channel assignment plan for a cellular communication system to include shared frequency channels.
As cellular communication has become more popular, cellular services providers have felt increasingly pressured to use the cellular radiofrequency (RF) spectrum as efficiently as possible. Greater efficiency allows a service provider to carry more calls using a given amount of RF spectrum. Accordingly, network planning tools exist for assisting network planners in devising channel assignment plans for cellular networks.
Typically, the development of a channel assignment plan begins with the determination of the amount of call traffic to be handled by each cell in the cellular network. Generally, the heaviest hour of cellular call traffic for a cell determines the quantity of frequency channels that should be assigned to that cell to achieve a desired grade of service. The channel assignment plan then assigns particular frequency channels to each cell so that each cell is desirably provided with the required number of channels, while at the same time meeting all of the constraints on frequency channel reuse.
A frequency channel assigned to one cell can be reused in another cell provided that it does not produce an unacceptably high level of interference in any of the cells, and provided that handoffs between the cells are not impaired. Both requirements can be satisfied by having a sufficient distance between cells using the same frequency. Since the number of frequency channels available for a given system is strictly limited, the degree to which frequencies can be reused within a system determines the amount of traffic that can be handled by the system.
When a cellular network is required to carry high volumes of traffic, it is typical for a cellular network planning process to produce an undesirable channel assignment plan. In an undesirable channel assignment, some cells have may have a deficiency in the quantity of channels assigned than are needed for the desired grade of service.
Some prior art systems attempt to mitigate the problems associated with these deficiencies in assigned channels by implementing channel borrowing techniques. One type of technique involves dynamic channel allocation. Dynamic channel techniques determine and change the amount of shared frequency channels assigned to cells in a cellular network based on current need.
A problem with dynamic channel allocation techniques relates to the physical complexity of the system employing such a technique. Specifically, the base station of each cell must be able to transmit and receive not only on the channels specifically allocated to that cell, but also on any of the channels that are allocated dynamically from, for example, a central pool of channels. This also causes additional complexity in the management of system channel resources.
Another type of channel borrowing involves utilizing channels assigned to neighboring cells. Thus, if a new call finds all channels assigned to the subject cell occupied, the call may not necessarily be blocked. Rather, if the user is also within range of a neighboring cell""s base station, the user will try to use a channel that was assigned to the neighboring cell""s base station. This link is established through the base station of the neighboring cell. This type of borrowing scheme utilizes the overlapping coverage areas of cellular base stations. With this type of borrowing technique, channels are not temporarily transferred from one base station to another, only the right to use a particular channel is transferred to a user in another cell.
An advantage of this type of channel borrowing is that each cellular base station need only accommodate the channels assigned to the base station itself. The simplicity of such an arrangement has accompanying problems. For example, users of borrowed channels must be in the region of coverage overlap provided by the adjacent cellular base station. As a result, the users tend to be relatively far from the cellular base stations through which they transmit and receive signals. Therefore, the quality of borrowed channel transmissions is lower than that of a cell""s regular assigned channels. By utilizing such a channel borrowing system, co-channel interference is also increased because the borrowed channels are used beyond their normal transmission range.
Accordingly, it is an advantage of the present invention that a system and method are provided for adapting a channel assignment plan to include shared frequency channels;
Another advantage is that the shared frequency channels are identified in the channel assignment plan during cellular network planning;
Yet another advantage of the present invention is that shared channel assignment is biased to favor assignment of shared frequency channels to those cells exhibiting the greatest deficiency in the quantity of channels assigned to them in the channel assignment plan.
The above and other advantages of the present invention are carried out in one form by a method for adapting a channel assignment plan to include shared frequency channels. The method is carried out in a cellular communication network having a channel assignment plan that distinguishes frequency channels assigned to a plurality of cells. The method calls for identifying a borrower cell from the plurality of cells, the borrower cell being assigned a first subset of the frequency channels, and the first subset having a deficiency in a quantity of the frequency channels assigned to the borrower cell. The method further calls for selecting a donor cell from the plurality of cells in response to a load factor for the donor cell, the donor cell being assigned a second subset of the frequency channels. A candidate frequency channel is chosen from the second subset. A quality measure is determined for the candidate frequency channel, and the candidate frequency channel is designated as one of the shared frequency channels in the channel assignment plan when the quality measure exceeds a channel sharing constraint.